Multi-piece solid golf ball

ABSTRACT

A golf ball having a core, a cover and at least one intermediate layer therebetween satisfies specific conditions concerning the relative surface hardnesses of the core, the intermediate layer-encased sphere and the ball, and has a specific core hardness profile. Also, the value V/H, where V is the initial velocity of the ball and H is the deflection of the ball under specific loading, is set within a specific range. The ball has a construction which, when used by the ordinary amateur golfer, reduces the spin rate on full shots, enabling good distances to be obtained both on shots with a driver and on shots with an iron. The ball also has a soft yet crisp feel at impact.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under U.S.C. §119(a) on Patent Application No. 2015-209568 filed in Japan on Oct. 26, 2015, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball which has a core of at least one layer, a cover of at least one layer, and at least one intermediate layer between the core and the cover.

BACKGROUND ART

Recently, a variety of golf balls have been proposed that set out to achieve certain intended spin properties and an increased distance, both by imparting the ball with a multilayer structure and also by designing a cover that is soft on the inside and hard on the outside, i.e., a cover having an outermost layer which is harder than an intermediate layer. Such golf balls are described in, for example, JP Nos. 3505922, 3533953, 3575524, 3661831, 3575525, 3428454, 3468153, 3685245, 3685248, 3772252, 4092532, 5042455 and 5445620.

In addition to the above art, four-piece solid golf balls have been proposed in which the ball construction includes a three-layer cover consisting of an envelope layer, an intermediate layer and an outermost layer that are formed in such a way that the intermediate layer is harder than the envelope layer and the outermost layer is harder than the intermediate layer. Such golf balls are described in JP Nos. 3304891 and 3304892. In addition, spin-type golf balls that are hard on the inside and soft on the outside have also been proposed, which balls have an outermost layer that is made of urethane and is softer than an intermediate layer. Such golf balls are described in JP-A 2015-077405 and JP-A 2015-047502.

However, in these prior-art golf balls, the hardness profile of the core is not sufficiently optimized. In particular, for the general user and amateur golfer, there remains room for improvement in terms of increasing the distance by reducing the spin rate on full shots. Also, such golf balls have an inadequate distance on shots taken with a middle iron, in addition to which there is room for improvement in the feel of the ball and its durability to repeated impact.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a multi-piece solid golf ball which, for the general user and amateur golfer, achieves a good distance not only on shots with a driver (W#1), but also on shots with a middle iron, which has a good feel at impact that is both soft and crisp, and which moreover has an excellent durability to cracking on repeated impact.

As a result of extensive investigations, we have discovered that, in a multi-piece solid golf ball having a core, a cover and at least one intermediate layer therebetween, by adjusting within specific ranges the value obtained by subtracting the surface hardness of an intermediate layer-encased sphere from the surface hardness of the ball, the value obtained by subtracting the surface hardness of the core from the surface hardness of the intermediate layer-encased sphere, and the hardnesses at specific positions in the core hardness profile—these being the core center, a position 5 mm from the core center, a position 10 mm from the core center, a position 15 mm from the core center and the core surface, by adjusting within a specific range the value obtained by subtracting the core center hardness from the core surface hardness, and by also adjusting within a specific range the value of V/H, where V is the ball initial velocity (m/s) and H is the ball deflection (mm) when compressed under a final load of 1,275 N from an initial load of 98 N, particularly for the general user and amateur golfer, a good distance can be obtained, not only on shots with a driver (W#1) but also on shots with a middle iron, a good feel at impact that is both soft and yet crisp and solid can be obtained, and moreover the durability to cracking on repeated impact is excellent.

Accordingly, the invention provides a multi-piece solid golf ball having a core, a cover, and at least one intermediate layer therebetween, wherein the value obtained by subtracting a surface hardness of an intermediate layer-encased sphere from a surface hardness of the ball, expressed in terms of Shore D hardness, is from 7 to 15; the value obtained by subtracting a surface hardness of the core from the surface hardness of the intermediate layer-encased sphere, expressed in terms of Shore D hardness, is within ±6; and, in a core hardness profile, the JIS-C hardness at a center of the core is 55±5, the JIS-C hardness at a position 5 mm from the core center is 57±5, the JIS-C hardness at a position 10 mm from the core center is 57±5, the JIS-C hardness at a position 15 mm from the core center is 70±5, and the JIS-C hardness at the core surface is 79±5. In addition, the value obtained by subtracting the core center hardness from the core surface hardness, expressed in terms of JIS-C hardness, is at least 22. Also, letting V be the initial velocity (m/s) of the ball and H be the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N, the value V/H is from 18 to 24 m/s·mm⁻¹.

In a preferred embodiment of the golf ball of the invention, the value obtained by subtracting the surface hardness of the intermediate layer-encased sphere from the surface hardness of the ball, expressed in terms of Shore D hardness, is from 8 to 13.

In another preferred embodiment of the inventive golf ball, in the core hardness profile, the JIS-C hardness at the core center is 55±3, the JIS-C hardness at a position 5 mm from the core center is 57±3, the JIS-C hardness at a position 10 mm from the core center is 57±3, the JIS-C hardness at a position 15 mm from the core center is 70±3, and the JIS-C hardness at the core surface is 79±3.

Alternatively, in the core hardness profile, the JIS-C hardness at the core center may be 55±2, the JIS-C hardness at a position 5 mm from the core center may be 57±2, the JIS-C hardness at a position 10 mm from the core center may be 57±2, the JIS-C hardness at a position 15 mm from the core center may be 70±2, and the JIS-C hardness at the core surface may be 79±2.

In yet another preferred embodiment, the core hardness profile satisfies the conditions:

0<C10−Cc≦8,  (1)

C10−Cc<Cs−C10, and  (2)

15<Cs−C10,  (3)

where Cc is the JIS-C hardness at the core center, C10 is the JIS-C hardness at a position 10 mm from the core center, and Cs is the JIS-C hardness at the core surface.

In a further preferred embodiment, the golf ball satisfies the condition:

PS ₇ /S/H×100≧6.20 (mm⁻¹),  (4)

where PS₇ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface when the ball is subjected to a load of 6,864 N, S is the hypothetical planar area (mm²), defined as the area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N.

In a still further preferred embodiment, the golf ball satisfies the condition:

PS ₂ /S/H×100≧1.85 (mm⁻¹),  (5)

where PS₂ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface when the ball is subjected to a load of 1,961 N, S is the hypothetical planar area (mm²), defined as the area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N.

Advantageous Effects of the Invention

The multi-piece golf ball of the invention has a construction which, when used by the ordinary amateur golfer, reduces the spin rate on full shots, enabling a good distance to be achieved on shots with a driver (W#1) and also on shots with an iron. In addition, the ball has a feel on impact that is both soft and yet crisp and solid. Moreover, the ball maintains a good level of durability to cracking on repeated impact.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIGS. 1A and 1B are enlarged cross-sectional diagrams of a dimple on the golf balls used in Working Examples 1 and 2.

FIGS. 2A and 2B are enlarged cross-sectional diagrams of dimples on the golf ball used in Working Example 3.

FIGS. 3A and 3B show explanatory diagrams for a method of determining the pressed area of a golf ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the foregoing diagrams.

The multi-piece solid golf ball of the invention has a core, a cover, and at least one intermediate layer between the core and the cover.

The core diameter, although not particularly limited, is typically from 36.7 to 38.7 mm, preferably from 37.1 to 38.3 mm, more preferably from 37.3 to 38.1 mm, and even more preferably from 37.5 to 37.9 mm. When the core diameter is too small, the initial velocity of the ball on shots with a driver (W#1) is low, as a result of which the intended distance may not be obtained. On the other hand, when the core diameter is too large, the durability to cracking on repeated impact may worsen or the spin rate-lowering effect on full shots may be inadequate, as a result of which the intended distance may not be obtained.

The core has a deflection when compressed under a final load of 1,275 N from an initial load of 98 N which, although not particularly limited, is preferably from 4.1 to 5.5 mm, more preferably from 4.3 to 5.0 mm, and even more preferably from 4.5 to 4.8 mm. When this value is too small, i.e., when the core is too hard, the spin rate may rise excessively, resulting in a poor distance, and the feel at impact may become too hard. On the other hand, when this value is too large, i.e., when the core is too soft, the rebound may be too low, resulting in a poor distance, the feel on impact may be too soft, and the durability to cracking on repeated impact may worsen.

The core center hardness (Cc) and the cross-sectional hardnesses at specific positions described below refer to hardnesses measured at the center and specific positions in a cross-section obtained by cutting the core in half through the center. The surface hardness (Cs) refers to the hardness measured at the spherical surface of the core.

The center hardness (Cc) of the core, expressed in terms of JIS-C hardness, is from 50 to 60, preferably from 52 to 58, and more preferably from 53 to 57. The JIS-C hardness at a position 5 mm from the core center (C5) is from 52 to 62, preferably from 54 to 60, and more preferably from 55 to 59. The JIS-C hardness at a position 10 mm from the core center (C10) is from 52 to 62, preferably from 54 to 60, and more preferably from 55 to 59. When these hardness values are too large, the spin rises excessively, as a result of which a good distance is not obtained, or the feel at impact is hard. On the other hand, when these hardness values are too small, the durability to cracking on repeated impact worsens, or the feel at impact is too soft.

The JIS-C hardness at a position 15 mm from the core center (C15) is from 65 to 75, preferably from 67 to 73, and more preferably from 68 to 72. The surface hardness (Cs) of the core, expressed in terms of JIS-C hardness, is from 74 to 84, preferably from 76 to 80, and more preferably from 77 to 81. When these hardness values are too large, the feel at impact is hard and the durability to cracking on repeated impact worsens. On the other hand, when these hardness values are too small, the spin rate rises excessively and the rebound decreases, resulting in a poor distance.

The Cs−Cc value obtained by subtracting the core center hardness from the core surface hardness, expressed in terms of JIS-C hardness, is at least 22, preferably from 23 to 30, and more preferably from 24 to 26. When this hardness difference is too small, the spin rate rises excessively and a good distance is not achieved.

The C10−Cc value is preferably from 0 to 8, more preferably from 1 to 6, and even more preferably from 2 to 4. This value means that up to about 10 mm from the core center, the core profile does not have a very steep gradient. Outside of this range in values, the spin rate on full shots may become high, as a result of which the intended distance may not be obtained, or the durability to cracking on repeated impact may worsen.

The Cs−C10 value is preferably larger than the C10−Cc value. This signifies that, in the core hardness profile, the outer portion of the core more than 10 mm from the core center has a steeper hardness gradient than the inner portion of the core up to 10 mm from the core center. When the Cs −C10 value is smaller than the C10−Cc value, the spin rate on full shots may rise, as a result of which the intended distance may not be achieved.

The value Cs−C10 value is preferably from 10 to 30, more preferably from 13 to 25, and even more preferably from to 22. This value signifies that the gradient in the core hardness profile becomes steep when the hardness difference between the position 10 mm from the core center and the core surface, expressed in terms of JIS-C hardness, exceeds 15. When this value is outside of the foregoing range, the spin rate on full shots may rise, as a result of which a good distance may not be obtained.

The core having the above hardness profile and deflection can be made of a material that is composed primarily of rubber. For example, use may be made of a rubber composition prepared by compounding a base rubber as the chief component and, together with this, other ingredients such as a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound.

It is preferable to use polybutadiene as the base rubber. The polybutadiene has a cis-1,4 bond content on the polymer chain of typically at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. When the content of cis-1,4 bonds among the bonds on the polybutadiene molecule is too low, the resilience may decrease.

Rubber components other than this polybutadiene may be included in the base rubber within a range that does not detract from the advantageous effects of the invention. Examples of such rubber components other than the foregoing polybutadiene include other polybutadienes, and diene rubbers other than polybutadiene, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

The organic peroxide used in the invention is not particularly limited, although the use of an organic peroxide having a one-minute half-life temperature of 110 to 185° C. is preferred. One, two or more organic peroxides may be used. The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, and even more preferably not more than 3 parts by weight.

The co-crosslinking agent is exemplified by unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids. Illustrative examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, and are exemplified by those obtained by neutralizing the foregoing unsaturated carboxylic acids with the desired metal ions. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

These unsaturated carboxylic acids and/or metal salts thereof are included in an amount per 100 parts by weight of the base rubber which is typically at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight. The upper limit is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than parts by weight, and most preferably not more than 40 parts by weight. When too much is included, the feel of the ball may become too hard and unpleasant. When too little is included, the rebound may decrease.

In order to have the core satisfy the desired hardness profile described above, water or a water-containing material may be added when compounding the various ingredients of the core-forming rubber composition. Decomposition of the organic peroxide within the core formulation can be promoted by the direct addition of water (or a water-containing material) to the core material. It is known that the decomposition efficiency of the organic peroxide within the core-forming rubber composition changes with temperature and that, starting at a given temperature, the decomposition efficiency rises with increasing temperature. If the temperature is too high, the amount of decomposed radicals rises excessively, leading to recombination between radicals and, ultimately, deactivation. As a result, fewer radicals act effectively in crosslinking. Here, when a heat of decomposition is generated by decomposition of the organic peroxide at the time of core vulcanization, the vicinity of the core surface remains at substantially the same temperature as the temperature of the vulcanization mold, but the temperature near the core center becomes considerably higher than the mold temperature due to the build-up of heat of decomposition by the organic peroxide which has decomposed from the outside. In cases where water (or a water-containing material) is added directly to the core, because the water acts to promote decomposition of the organic peroxide, radical reactions like those described above can be made to differ at the core center and at the core surface. That is, decomposition of the organic peroxide is further promoted near the center of the core, bringing about greater radical deactivation, which leads to a further decrease in the amount of active radicals. As a result, it is possible to obtain a core in which the crosslink densities at the core center and the core surface differ markedly. It is also possible to obtain a core having different dynamic viscoelastic properties at the core center. Along with achieving a lower spin rate, golf balls having such a core are also able to exhibit excellent durability and undergo less change over time in rebound. When zinc monoacrylate is used instead of the above water, water is generated from the zinc monoacrylate by heat during kneading of the compounding materials. An effect similar to that obtained by the addition of water can thereby be obtained.

The water used here is not particularly limited, and may be distilled water or tap water. The use of distilled water which is free of impurities is especially preferred. The amount of water included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, and more preferably at least 0.3 part by weight. The upper limit is preferably not more than 5 parts by weight, and more preferably not more than 4 parts by weight.

Alternatively, a metal monocarboxylate may be used instead of the above-described water. Metal monocarboxylates, in which the carboxylic acid is presumably coordination-bonded to the metal, are distinct from metal dicarboxylates such as zinc diacrylate of the formula (CH₂═CHCOO)₂Zn. A metal monocarboxylate introduces water into the rubber composition by way of a dehydration/condensation reaction, and thus provides an effect similar to that of water. Moreover, because a metal monocarboxylate can be added to the rubber composition as a powder, the operations can be simplified and uniform dispersion within the rubber composition is easy. A monosalt is required in order to carry out the above reaction effectively. The amount of metal monocarboxylate included per 100 parts by weight of the base rubber is preferably at least 1 part by weight, and more preferably at least 3 parts by weight. The upper limit in the amount of metal monocarboxylate included is preferably not more than 60 parts by weight, and more preferably not more than 50 parts by weight. When the amount of metal monocarboxylate included is too small, it may be difficult to obtain a suitable crosslink density and tan δ, as a result of which a sufficient golf ball spin rate-lowering effect may not be achievable. On the other hand, when too much is included, the core may become too hard, as a result of which it may be difficult for the ball to maintain a suitable feel at impact.

The carboxylic acid used may be, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid or stearic acid. Examples of the substituting metal include sodium, potassium, lithium, zinc, copper, magnesium, calcium, cobalt, nickel and lead, although the use of zinc is preferred. Illustrative examples of the metal monocarboxylate include zinc monoacrylate and zinc monomethacrylate, with the use of zinc monoacrylate being especially preferred.

Core production may be carried out in the usual manner by molding a spherical molded article (core) using heat and compression under vulcanization conditions of at least 140° C. and not more than 180° C. and at least 10 minutes and not more than 60 minutes.

Next, the intermediate layer is described.

The intermediate layer has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 42 to 52, more preferably from 44 to 50, and even more preferably from 46 to 48. The sphere consisting of the core encased by the intermediate layer, referred to herein as the “intermediate layer-encased sphere,” has a surface hardness, expressed in terms of Shore D hardness, which is preferably from 48 to 58, more preferably from 50 to 56, and even more preferably from 52 to 54. When the intermediate layer is too soft, the spin rate on full shots may rise excessively, as a result of which a good distance may not be achieved. On the other hand, when the intermediate layer is too hard, the durability to cracking on repeated impact may worsen and the feel of the ball on impact may become too hard.

The intermediate layer has a thickness which, although not particularly limited, is preferably from 1.0 to 1.5 mm, more preferably from 1.1 to 1.4 mm, and even more preferably from 1.2 to 1.3 mm. When the intermediate layer thickness falls outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and a good distance may not be achieved.

The intermediate layer material is not particularly limited, although preferred use can be made of various thermoplastic resin materials. In particular, to fully achieve the desired effects of the invention, it is preferable to use a high-resilience resin material as the intermediate layer material. For example, the use of an ionomer resin material or the subsequently described highly neutralized resin material is preferred.

Specifically, a molded material obtained by molding a resin composition of the components (I) to (IV) described below under applied heat may be used as the highly neutralized resin material.

Preferred use can be made of the two following components (I) and (II) as the base resins:

-   (I) An olefin-unsaturated carboxylic acid-unsaturated carboxylic     acid ester terpolymer, or a metal neutralization product thereof,     having a weight-average molecular weight (Mw) of at least 140,000,     an acid content of 10 to 15 wt % and an ester content of at least 15     wt %; and -   (II) An olefin-acrylic acid random copolymer, or a metal     neutralization product thereof, having a weight-average molecular     weight (Mw) of at least 140,000 and an acid content of 10 to 15 wt     %.

By thus making these molecular weights large, the resin material can be assured of having sufficient resilience.

It is thought that because the acid components and ester contents of the respective copolymers serving as the base resins (I) and (II) differ, these two types of base resins interlock in a complex manner, giving rise to molecular synergistic effects that can increase the rebound and durability of the ball. That is, by specifying the weight-average molecular weight, acid content and ester content as indicated above in such a way as to select a material that is relatively soft as the terpolymer serving as base resin (I), and by specifying the type of acid, weight-average molecular weight and acid content in such a way as to select a relatively hard material as base resin (II), it is possible with a blend of these polymers to ensure sufficient resilience and durability for use as a golf ball material.

As noted above, copolymers or ionomers with weight-average molecular weights (Mw) set in specific ranges are used as components (I) and (II). Illustrative examples of commercial products that may be used for this purpose include the Nucrel series (DuPont-Mitsui Polychemicals Co., Ltd.), the Escor series (ExxonMobil Chemical), the Surlyn series (E.I. DuPont de Nemours & Co.), and the Himilan series (DuPont-Mitsui Polychemicals Co., Ltd.).

In addition, (III) a basic inorganic metal compound is preferably included as a component for neutralizing acid groups in above components (I) and (II) and subsequently described component (IV). By even more highly neutralizing the resin material in this way, the spin rate of the ball on full shots is even further reduced without adversely affecting the feel of the ball, thus making an increased distance fully achievable. Illustrative examples of the metal ions in the basic inorganic metal compound include Na⁺, K⁺, Li⁺, Zn²⁺, Ca²⁺, Mg²⁺, Cu⁺ and Co²⁺. Of these, Na⁺, Zn²⁺, Ca²⁺ and Mg²⁺ are preferred, and Mg²⁺ is more preferred. These metal salts may be introduced into the resin using, for example, formates, acetates, nitrates, carbonates, bicarbonates, oxides and hydroxides.

This basic inorganic metal compound (III) is included in the resin composition in an amount equivalent to at least 70 mol %, based on the acid groups in the resin composition. Here, the amount in which the basic inorganic metal compound serving as component (III) is included may be selected as appropriate for obtaining the desired degree of neutralization. Although this amount depends also on the degree of neutralization of the base resins (components (I) and (II)) that are used, in general it is preferably from 1.0 to 2.5 parts by weight, more preferably from 1.1 to 2.3 parts by weight, and even more preferably from 1.2 to 2.0 parts by weight, per 100 parts by weight of the combined amount of the base resins (components (I) and (II)). The degree of neutralization of the acid groups in components (I) to (IV) is preferably at least 70 mol %, more preferably at least 90 mol %, and even more preferably at least 100 mol %.

Next, the anionic surfactant serving as component (IV) is described. The reason for including an anionic surfactant is to improve the durability after resin molding while ensuring good flowability of the overall resin composition. The anionic surfactant is not particularly limited, although the use of one having a molecular weight of from 140 to 1,500 is preferred. Exemplary anionic surfactants include carboxylate surfactants, sulfonate surfactants, sulfate ester surfactants and phosphate ester surfactants. Preferred examples include one or more selected from the group consisting of various fatty acids such as stearic acid, behenic acid, oleic acid and maleic acid, derivatives of these fatty acids, and metal salts thereof. Selection from the group consisting of stearic acid, oleic acid and mixtures thereof is especially preferred. Alternatively, exemplary organic acid metal salts that may serve as component (IV) include metal soaps, with the metal salt being one in which a metal ion having a valence of 1 to 3 is used. The metal is preferably selected from the group consisting of lithium, sodium, magnesium, aluminum, potassium, calcium and zinc, with the use of metal salts of stearic acid being especially preferred. Specifically, the use of magnesium stearate, calcium stearate, zinc stearate or sodium stearate is preferred.

Component (IV) is included in an amount, per 100 parts by weight of the base resins serving as components (I) and (II), of 1 to 100 parts by weight, preferably 10 to 90 parts by weight, and more preferably 20 to 80 parts by weight. When the component (IV) content is too low, it may be difficult to lower the hardness of the resin material. On the other hand, at a high content, the resin material is difficult to mold and bleeding at the material surface increases, adversely affecting the molded article.

In this invention, the moldability of the material and the productivity can be further increased by suitably adjusting the compounding ratio between components (III) and (IV). When the content of the basic inorganic metal compound serving as component (III) is too high, the amount of gases such as organic acids that evolve during molding decreases, but the flowability of the material diminishes. Conversely, when the content of component (III) is low, the amount of gases generated increases. On the other hand, when the content of the anionic surfactant serving as component (IV) is too high, the amount of gas consisting of fatty acids and other organic acids increases during molding, which has a large impact in terms of molding defects and productivity. Conversely, when the content of component (IV) is low, the amount of gases generated decreases, but the flowability and durability decline. Therefore, achieving a proper compounding balance between components (III) and (IV) is also important. Specifically, it is desirable to set the compounding ratio between components (III) and (IV), expressed as the weight ratio (III):(IV), to from 4.0:96.0 to 1.0:99.0, and especially from 3.0:97.0 to 1.5:98.5.

The resin composition of above components (I) to (IV) accounts for preferably at least 50 wt %, more preferably at least 60 wt %, even more preferably at least 70 wt %, and most preferably at least 90 wt %, of the total amount of the intermediate layer material.

A non-ionomeric thermoplastic elastomer may be included in the intermediate layer material. The non-ionomeric thermoplastic elastomer is preferably included in an amount of from 1 to 50 parts by weight per 100 parts by weight of the combined amount of the base resins.

The non-ionomeric thermoplastic elastomer is exemplified by polyolefin elastomers (including polyolefins and metallocene-catalyzed polyolefins), polystyrene elastomers, diene polymers, polyacrylate polymers, polyamide elastomers, polyurethane elastomers, polyester elastomers and polyacetals.

Illustrative examples of highly neutralized resin materials containing above components (I) to (IV) include the products available under the trade names HPF 1000, HPF 2000 and HPF AD1027, as well as the experimental material HPF SEP1264-3, all produced by E.I. DuPont de Nemours & Co.

Optional additives may be suitably included in the intermediate layer material according to the intended use. For example, various additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When such additives are included, the content thereof per 100 parts by weight of components (I) to (IV) combined is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight, with the upper limit being preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

It is desirable to abrade the surface of the intermediate layer in order to increase adhesion with the polyurethane that is preferably used in the subsequently described cover (outermost layer). In addition, it is desirable to apply a primer (adhesive) to the surface of the intermediate layer following such abrasion treatment or to add an adhesion reinforcing agent to the intermediate layer material.

The intermediate layer material has a specific gravity which is typically less than 1.1, preferably from 0.90 to 1.05, and more preferably from 0.93 to 0.99. Outside of this range, the rebound becomes small, as a result of which a good distance may not be obtained, or the durability to cracking on repeated impact may worsen.

The value obtained by subtracting the core surface hardness from the surface hardness of the intermediate layer-encased sphere, expressed in terms of Shore D hardness, must be within ±6, and is preferably within ±4, and more preferably within ±2. When this value is too large, the feel at impact is too hard, or the durability to cracking under repeated impact worsens. On the other hand, when this value is too small, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained.

Next, the cover, which is the outermost layer of the ball, is described.

The cover (outermost layer) has a material hardness expressed in terms of Shore D hardness which, although not particularly limited, is preferably from 51 to 61, more preferably from 53 to 59, and even more preferably from 55 to 57.

The cover-encased sphere, i.e., the ball, has a surface hardness expressed in terms of Shore D hardness which is preferably from 57 to 67, more preferably from 60 to 65, and even more preferably from 61 to 63. When the surface hardness of the ball is softer than this range, the spin rate on driver (W#1) shots and iron shots may become too high, as a result of which the desired distance may not be obtained. When the surface hardness of the ball is higher than this range, the durability to cracking on repeated impact may worsen or the feel at impact may be too hard.

The cover serving as the outermost layer of the ball has a thickness which, although not particularly limited, is preferably from 1.0 to 1.5 mm, more preferably from 1.1 to 1.4 mm, and even more preferably from 1.2 to 1.3 mm. When the cover thickness falls outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate, as a result of which a good distance may not be obtained.

The value obtained by subtracting the cover thickness from the above-described intermediate layer thickness is preferably from −1.0 to 1.0 mm, more preferably form −0.6 to 0.5 mm, and even more preferably from −0.3 to 0 mm. When this value is too large or too small, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained.

The cover material is not particularly limited, although the use of an ionomer resin material is preferred.

The manufacture of multi-piece solid golf balls in which the above-described core, intermediate layer and cover (outermost layer) are formed as successive layers may be carried out by a customary method such as a known injection-molding process. For example, a multi-piece golf ball can be obtained by placing a core in a given injection mold, injecting an intermediate layer material over the core to give an intermediate sphere, and subsequently placing the resulting sphere in another injection mold and injection-molding a cover (outermost layer) material over the sphere. Alternatively, a cover (outermost layer) may be formed over the intermediate sphere by a method that involves encasing the intermediate sphere with a cover, this being carried out by, for example, enclosing the intermediate sphere within two half-cups that have been pre-molded into hemispherical shapes, and then molding under applied heat and pressure.

The value obtained by subtracting the surface hardness of the core from the surface hardness of the ball, expressed in terms of Shore D hardness, is preferably from 4 to 16, more preferably from 6 to 14, and even more preferably from 8 to 12. When this value is too large, the durability to cracking under repeated impact may worsen. On the other hand, when this value is too small, the spin rate on full shots may rise excessively, as a result of which the intended distance may not be obtained.

The value obtained by subtracting the surface hardness of the intermediate layer-encased sphere from the surface hardness of the ball, expressed in terms of Shore D hardness, is from 7 to 15, preferably from 8 to 13, and more preferably from 9 to 11. When this value is too large, the durability to cracking under repeated impact worsens. On the other hand, when this value is too small, the spin rate on full shots rises excessively, as a result of which the intended distance cannot be obtained.

Numerous dimples may be formed on the cover (outermost layer). The number of dimples arranged on the cover surface, although not particularly limited, may be set to preferably at least 250, and more preferably at least 300, with the upper limit being preferably not more than 500, and more preferably not more than 450.

The dimple surface coverage SR (i.e., the ratio of the sum of the individual dimple areas with respect to the total surface area of the hypothetical sphere were the ball assumed to have no dimples thereon) is set to preferably at least 70%, more preferably at least 75%, and even more preferably at least 80%. The maximum dimple surface coverage SR, although not particularly limited, is preferably not more than 99%. It is especially desirable for the ball to be provided with at least three types of dimples of differing size, and for the dimples to be thereby uniformly arranged on the spherical surface of the ball without leaving gaps.

The dimple volume occupancy VR (i.e., the sum of the volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, expressed as a ratio with respect to the volume of the hypothetical sphere were the ball assumed to have no dimples thereon) is set to preferably at least 0.75%, more preferably at least 0.80%, and even more preferably at least 1.1%. The upper limit in the dimple volume occupancy VR is preferably not more than 1.5%, and more preferably not more than 1.4%.

Although the dimple shapes are not particularly limited, by giving the base of a dimple a specific shape in which the center of the dimple curves upward toward the outside of the golf ball, the ball can be imparted with the subsequently described specific pressed area without a loss of the aerodynamic performance inherent to the dimples. In this dimple shape, the portion having an upwardly curved shape can, moreover, be given a flat shape in the central region thereof. Beveling the corner on the outer edge portion of this flat region can effectively increase the contact area when the ball is struck with a golf club.

The relationship between the pressed area, the hypothetical planar surface area and the deflection of the golf ball is preferably set within the following ranges.

The golf ball of the invention preferably satisfies the condition

PS ₇ /S/H×100≧5.70 (mm⁻¹)

and more preferably satisfies the condition

PS ₇ /S/H×100≧6.20 (mm⁻¹),

where PS₇ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface, when the ball is subjected to a load of 6,864 N (700 kgf), S is the hypothetical planar area (mm²), defined as the surface area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N.

By having the pressed area of the golf ball under loading on a driver shot by an ordinary golfer satisfy the above condition, the surface area of contact between the ball and golf club increases and frictional forces with the club rise, as a result of which the amount of back spin on driver shots decreases, enabling the distance to be improved.

The golf ball of the invention also preferably satisfies the condition

PS ₂ /S/H×100≧1.70 (mm⁻¹)

and more preferably satisfies the condition

PS ₂ /S/H×100≧1.85 (mm⁻¹)

where PS₂ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface, when the ball is subjected to a load of 1,961 N (200 kgf), S is the hypothetical planar area (mm²), defined as the surface area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N.

By having the pressed area of the golf ball under loading on an approach shot by an ordinary golfer satisfy the above condition, the surface area of contact between the ball and golf club increases and frictional forces with the club rise, as a result of which the amount of back spin on approach shots increases, enabling movement of the ball to be stopped in a straighter line near the landing point of the ball.

The hypothetical planar surface area S of the golf ball is determined by the ball diameter. The diameter may be set in conformity with the Rules of Golf for play, this being of a size such that the ball does not pass through a ring having an inside diameter of 42.672 mm and is not more than 42.80 mm.

The pressed areas PS₇ and PS₂ of the golf ball under predetermined loads represent the areas of contact by the golf ball with the golf club at the time of given shots. These areas of contact can be made larger than in the prior art by means of the dimple structure. However, the pressed area PS is dependent on the size of the golf ball, becoming larger when the size of the golf ball is larger and smaller when the size of the golf ball is smaller. Accordingly, by dividing the pressed area by the hypothetical planar surface area S and expressing the result as a percentage, it is possible to evaluate the increase in the area of contact due to the dimple construction without being influenced by the size of the golf ball. The pressed area PS is also dependent on the deflection H of the golf ball, becoming larger when the deflection H is larger, and smaller when the deflection H is smaller. Therefore, by dividing the pressed area by the deflection H, it is possible to evaluate the increase in the area of contact due to the dimple construction without being influenced by the amount of golf ball deflection. Measurement of the pressed area may be carried out by, for example, placing pressure-sensitive paper on a flat surface, setting the golf ball to be tested on the paper, applying a load of 6,864 N or 1,961 N to the golf ball, and measuring the total area of the portion of the pressure-sensitive paper that has become colored as a result of contact with the golf ball. FIG. 3A shows an example of pressure-sensitive paper that was actually colored when a load of 6,864 N was applied to a golf ball, and FIG. 3B shows an example of pressure-sensitive paper that was actually colored when a load of 1,961 N (200 kgf) was applied to the same golf ball. In these diagrams, the round areas are dimples, and the solid (blackened) places indicate the colored portions. The area of the colored portions can be easily determined using a commercial pressure image analysis system.

The ball has a deflection (mm) when compressed under a final load of 1,275 N from an initial load of 98 N which, although not particularly limited, is preferably from 3.0 to 4.5 mm, more preferably from 3.2 to 4.0 mm, and even more preferably from 3.5 to 3.8 mm. When this value is too large, the feel at impact may be too soft, and the durability to cracking on repeated impact may worsen. On the other hand, when this value is too small, the feel at impact may be too hard and the spin rate on full shots may rise, as a result of which the intended distance may not be obtained.

The golf ball has an initial velocity, as measured according to the standards in the Royal and Ancient Golf Club of St. Andrews (R&A) Rules of Golf, of preferably from 76.4 to 77.724 m/s, more preferably at least 76.7 m/s, and even more preferably at least 77.0 m/s. At an initial velocity in excess of 77.724 m/s, the ball violates the Rules of Golf, making it unfit for use as an official ball. On the other hand, when the ball initial velocity is too low, the intended distance on full shots may not be achieved. The ball initial velocity is measured using the apparatus and under the conditions described below in the “Examples” section.

Also, in this invention, letting V be the initial velocity (m/s) of the ball and H be the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N, it is critical that the value V/H be from 18 to 24 m/s·mm⁻¹. The value V/H is preferably from 19 to 23.5, and more preferably from 20 to 23. When this value is too low, the intended distance on shots with a driver (W#1) cannot be obtained. On the other hand, when this value is too large, the feel at impact becomes hard and the durability to cracking on repeated impact worsens.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for use as a game ball, and can be formed to a weight of preferably from 45.0 to 45.93 g.

EXAMPLES

The following Working Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Working Examples 1 to 3, Comparative Examples 1 to 6 Formation of Core

Solid cores for the respective Working Examples and Comparative Examples were produced by preparing the rubber compositions shown in Table 1, then vulcanizing and molding the compositions under the vulcanization conditions shown in the same table.

TABLE 1 Working Example Comparative Example 1 2 3 1 2 3 4 5 6 Core Polybutadiene A 80 80 80 80 80 80 80 80 80 formulation Polybutadiene B 20 20 20 20 20 20 20 20 20 (pbw) Zinc 29.5 32.1 32.1 32.1 29.5 32.1 32.1 32.1 24.3 acrylate Organic 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 peroxide (1) Organic 2.5 peroxide (2) Water 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Barium 21.7 20.6 20.6 20.6 21.7 20.6 20.6 14.4 sulfate (1) Barium 25 sulfate (2) Zinc oxide 4 4 4 4 4 4 4 4 4 Zinc salt of 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.4 pentachloro- thiophenol Vulcanization Temperature 155 155 155 155 155 155 155 155 155 conditions (° C.) Time (min.) 13 13 13 13 13 13 13 13 13

Details on each of the ingredients in Table 1 are given below.

-   Polybutadiene A: Available under the trade name “BR 01” from JSR     Corporation -   Polybutadiene B: Available under the trade name “BR 51” from JSR     Corporation -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd. -   Organic peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic peroxide (2): A mixture of 1,1-di(t-butylperoxy)-cyclohexane     and silica, available under the trade name “Percumyl C-40” from NOF     Corporation -   Antioxidant: 2,2-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Barium sulfate (1): Available under the trade name “Barico #300”     from Hakusui Tech -   Barium sulfate (2): Available under the trade name “Precipitated     Barium Sulfate 100” from Sakai Chemical Co., Ltd. -   Zinc oxide: Available under the trade name “Zinc Oxide Grade 3” from     Sakai Chemical Co., Ltd. -   Zinc salt of pentachlorothiophenol:     -   Available from ZHEJIANG CHO & FU CHEMICAL. -   Water: Distilled water,     -   from Wako Pure Chemical Industries, Ltd.

Formation of Intermediate Layer and Cover

In each Example, an intermediate layer material formulated as shown in Table 2 was injected-molded over the core obtained above, thereby giving an intermediate layer-encased sphere. Next, using the cover material formulated as shown in Table 2, a cover (outermost layer) was injection-molded over the intermediate layer-encased sphere, thereby producing a golf ball having an intermediate layer and a cover (outermost layer) over the core.

TABLE 2 Resin materials (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 HPF 2000 100 Surlyn ® 8940 50 50 Surlyn ® 9945 7 Surlyn ® 9320 43 Surlyn ® 9910 50 Surlyn ® 7930 37 Surlyn ® 6320 35.5 Nucrel ® AN4318 27.5 Nucrel ® AN4319 20 Nucrel ® AN4221C 80 Titanium oxide 4 4 4 Magnesium stearate 60 Calcium hydroxide 1.5 Magnesium oxide 1 Polytail H 8 Hytrel ® 4047 100

Details on the materials shown in Table 2 are as follows.

-   HPF 2000: Available from E.I. DuPont de Nemours & Co. -   Surlyn®: Ionomers available from E.I. DuPont de Nemours & Co. -   Nucrel®: Ethylene-methacrylic acid copolymers available from     DuPont-Mitsui Polychemicals Co., Ltd. -   Magnesium stearate: “Magnesium Stearate G” from NOF Corporation     Calcium hydroxide: “Calcium Hydroxide CLS-B” from Shiraishi Calcium     Kaisha, Ltd. -   Magnesium oxide: “Kyowamag MF 150”     -   from Kyowa Chemical Industry Co., Ltd. -   Polytail H: Available from Mitsubishi Chemical Corporation -   Hytrel® 4047: A polyester elastomer available from DuPont-Toray Co.,     Ltd.

Dimples having the parameters shown in Table 3 below were formed at this time on the cover surface in the respective Working Examples and Comparative Examples. Six types of dimples of differing diameters as shown in Table 3 were arranged on the golf balls in each of the Working Examples and Comparative Examples, and set to the same surface coverage ratio SR.

TABLE 3 No. Number of dimples Diameter (mm) SR (%) 1 12 4.6 81 2 234 4.4 3 60 3.8 4 6 3.5 5 6 3.4 6 12 2.6 Total 330

Dimple Definitions

-   Diameter: Diameter of flat plane circumscribed by edge of dimple     (mm). -   SR: Sum of individual dimple areas as a percentage of the total     surface area of a hypothetical sphere were the golf ball to have no     dimples thereon (unit: %)

Two dimple shapes were used. Dimple A (FIG. 1) was used in Working Examples 1 and 2 and Comparative Examples 1 to 6. Dimple B (FIG. 2) was used only in Working Example 3. Of the six types of dimples of differing diameter in Table 3, the structures of the typical dimples having a diameter of 4.4 mm were as follows.

Dimple A

In the cross-sectional shape in FIG. 1, the depth L at the deepest point is 0.150 mm.

Dimple B

In the cross-sectional shape in FIG. 2, the depth H at the center point C is 0.097 mm, the depth D at the deepest point is 0.131 mm, the distance from the outer peripheral edge E to the position of the deepest point, relative to an arbitrary distance of 100 from the outer peripheral edge E to the center point C, is 39, the radius of curvature R is 0.5 mm and the edge angle A2 is 10.5°.

For each of the resulting golf balls, characteristics such as the core hardness profile, thicknesses and material hardnesses of the respective layers, and the surface hardnesses of various layer-encased spheres were evaluated by the methods described below. The results are shown in Table 4.

Core Hardness Profile

The indenter of a durometer was set so as to be substantially perpendicular to the spherical surface of the core, and the core surface hardness in terms of JIS-C hardness was measured as specified in JIS K6301-1975.

To obtain the cross-sectional hardnesses at the center and other specific positions of the core, the core was hemispherically cut so as form a planar cross-section, and measurements were carried out by pressing the indenter of a durometer perpendicularly against the cross-section at the measurement positions. These hardnesses are indicated as JIS-C hardness values.

The Shore D hardness at the core surface was measured with a type D durometer in accordance with ASTM D2240-95.

Diameters of Core and Intermediate Layer-Encased Sphere

The diameters at five random places on the surface were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single core or intermediate layer-encased sphere, the average diameter for five measurement specimens was determined.

Ball Diameter

The diameters at five random dimple-free areas on the surface of a ball were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for five measured balls was determined.

Deflections of Core and Ball

A core or ball was placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N from an initial load of 98 N was measured. The amount of deflection here refers in each case to the measured value obtained after holding the test specimen isothermally at 23.9° C.

Initial Velocity of Ball

The initial velocity was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The core was tested in a chamber at a room temperature of 23.9±2° C. after being held isothermally in a 23.9±1° C. environment for at least 3 hours. Each ball was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). One dozen balls were each hit four times. The time taken for the ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s). This cycle was carried out over a period of about 15 minutes.

Material Hardnesses of Intermediate Layer and Cover (Shore D Hardnesses)

The intermediate layer and cover-forming resin materials were molded into sheets having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardnesses were measured in accordance with ASTM D2240-95.

Surface Hardnesses of Intermediate Layer-Encased Sphere and Ball (Shore D Hardnesses)

Measurements were taken by pressing the durometer indenter perpendicularly against the surface of the intermediate layer-encased sphere or ball (i.e., the surface of the cover). The surface hardness of the ball (cover) is the measured value obtained at dimple-free places (lands) on the ball surface. The Shore D hardnesses were measured with a type D durometer in accordance with ASTM D2240-95.

Pressed Area

Measurement of the pressed area PS of a golf ball was carried out by placing pressure-sensitive paper (Prescale pressure measurement film for medium pressure, available from Fujifilm Corporation) on a flat surface, and setting a golf ball from the respective Working Examples and Comparative Examples thereon. Next, using a model 4204 tester from Instron Corporation, loads of 6,864 N (700 kgf) and 1,961 N (200 kgf) were applied to these golf balls, and the total area of the portion of the pressure-sensitive paper that became colored due to contact with the golf ball was measured. The area of the colored portion was determined using the FPD-9270 Prescale Pressure Image Analysis System (Fujifilm Corporation). In each case, the pressed area is the result of measurement at a single arbitrary position on the golf ball.

TABLE 4 Working Example Comparative Example 1 2 3 1 2 3 4 5 6 Construction 3-piece 3-piece 3-piece 3-piece 3-piece 3-piece 3-piece 3-piece 3-piece Core Diameter (mm) 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 37.7 Weight (g) 33.4 33.4 33.4 33.4 33.4 33.4 33.4 32.5 33.4 Specific gravity 1.19 1.19 1.19 1.19 1.19 1.19 1.19 1.16 1.19 Deflection (mm) 4.8 4.5 4.5 4.5 4.8 4.5 4.5 4.5 4.5 Hardnesss Center hardness, Cc (JIS-C) 54 55 55 55 54 55 55 55 58 profile Hardness at position 56 58 58 58 56 58 58 58 63 5 mm from center (JIS-C) Hardness at position 56 58 58 58 56 58 58 58 63 10 mm from center, C10 (JIS-C) Hardness at position 68 71 71 71 68 71 71 71 67 15 mm from center (JIS-C) Surface hardness, Cs (JIS-C) 78 80 80 80 78 80 80 80 77 Surface hardness (Shore D) 51 53 53 53 51 53 53 53 51 Surface − Center (Cs − Cc) 24 25 25 25 24 25 25 25 19 C10 − Cc 2 3 3 3 2 3 3 3 5 Cs − C10 22 22 22 22 22 22 22 22 14 Inter- Material No. 1 No. 1 No. 1 No. 1 No. 1 No. 5 No. 5 No. 6 No. 1 mediate Thickness (mm) 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 layer Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.96 1.12 0.96 Material hardness (Shore D) 47 47 47 47 47 55 55 40 47 Inter- Diameter (mm) 40.2 40.2 40.2 40.2 40.2 40.2 40.2 40.2 40.2 mediate Weight (g) 39.1 39.1 39.1 39.1 39.1 39.1 39.1 39.1 39.1 layer- Surface hardness (Shore D) 53 53 53 53 53 61 61 46 53 encased sphere Intermediate layer surface hardness − 2 0 0 0 2 8 8 −7 2 Core surface hardness (Shore D) Cover Material No. 2 No. 2 No. 2 No. 3 No. 4 No. 2 No. 4 No. 3 No. 2 Thickness (mm) 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 Specific gravity 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 0.97 Material hardness (Shore D) 56 56 56 50 63 56 63 50 56 Dimples A A B A A A A A A Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 Deflection H (mm) 3.75 3.5 3.5 3.7 3.55 3.4 3.25 3.8 3.6 Initial velocity V (m/s) 77.2 77.3 77.3 76.3 77.6 77.4 77.6 76.0 77.3 Initial velocity V/Deflection H 20.6 22.1 22.1 20.6 21.9 22.8 23.9 20.0 21.5 (m/s · mm⁻¹) Surface hardness (Shore D) 62 62 62 59 69 62 69 59 62 Ball surface hardness − 11 9 9 6 18 9 16 6 11 Core surface hardness (Shore D) Ball surface hardness − 9 9 9 6 16 1 8 13 9 Intermediate layer-encased sphere surface hardness (Shore D) Intermediate layer thickness − 0 0 0 0 0 0 0 0 0 Cover thickness (mm) S: Hypothetical planar area (mm²) 1432 1432 1432 1432 1432 1432 1432 1432 1432 PS7: Pressed area when loaded 311 293 322 308 297 286 275 315 300 at 6,864 N (mm²) PS2: Pressed area when loaded 95 89 96 94 90 87 83 96 91 at 1,961 N (mm²) Formula 1: PS7/S/H × 100 (mm⁻¹) 5.80 5.85 6.42 5.81 5.84 5.87 5.91 5.79 5.83 Formula 2: PS2/S/H × 100 (mm⁻¹) 1.76 1.77 1.92 1.77 1.77 1.78 1.78 1.76 1.77

In addition, the flight performance (W#1 and I#6), spin performance on approach shots, feel and durability of the golf balls obtained in the respective Examples of the invention and the Comparative Examples were evaluated according to the criteria indicated below. The results are shown in Table 5.

Flight Performance (W#1 Shots)

A driver (W#1) was mounted on a golf swing robot, and the distance traveled by the ball when struck at a head speed (HS) of 45 m/s was measured and rated according to the criteria shown below. The club used was a TourStage X-Drive709 D430 driver (2013 model, loft angle, 9.5°).

Rating Criteria:

-   -   Good: Total distance was 230.0 m or more     -   Fair: Total distance was at least 229.0 m, but less than 230.0 m     -   NG: Total distance was less than 229.0 m

Flight Performance (I#6 Shots)

A 6 iron (I#6) was mounted on a golf swing robot, and the distance traveled by the ball when struck at a head speed (HS) of 40 m/s was measured and rated according to the criteria shown below. The club used was a TourStage X-Blade707 (2012 model).

Rating Criteria:

-   -   Good: Total distance was 173.0 m or more     -   Fair: Total distance was at least 170.0 m, but less than 173.0 m     -   NG: Total distance was less than 170.0 m

Spin Performance on Approach Shots

A sand wedge was mounted on a golf swing robot, and the spin rate of the ball when hit at a head speed (HS) of 20 m/s was measured.

Feel

Sensory evaluations were carried out when the balls were hit with a driver (W#1) by amateur golfers having head speeds of 40 to 50 m/s. The feel of the ball was rated according to the following criteria.

Rating Criteria:

-   -   Good: Six or more out of ten golfers rated the feel as good     -   Fair: Three to five out of ten golfers rated the feel as good     -   NG: Two or fewer out of ten golfers rated the feel as good

Durability

A driver (W#1) was mounted on a golf swing robot, the ball was repeatedly struck at a head speed of 45 m/s, and the average value was measured for N=3 balls in each Example. Durability indexes for the balls in the respective Examples were calculated relative to an arbitrary index of 100 for the number of shots taken with the ball in Example 1 when the initial velocity of the ball fell to or below 97% of the average initial velocity for the first ten shots, and the durability was rated according to the following criteria.

Rating Criteria:

-   -   Good: Durability index was 95 or more     -   NG: Durability index was less than 95

TABLE 5 Working Example Comparative Example 1 2 3 1 2 3 4 5 6 Flight W#1 HS, Spin rate 2,485 2,580 2,512 2,790 2,326 2,675 2,575 2,735 2,687 performance 45 m/s (rpm) Total 230.4 230.1 230.6 226.1 232.2 230.0 231.5 226.8 228.2 distance (m) Rating good good good NG good good good NG NG I#6 Spin rate 4,101 4,338 4,210 4,688 3,956 4,513 4,221 4,695 4,503 (rpm) Total 175.9 173.4 174 168.1 177.7 171.0 174.1 168.5 171.4 distance (m) Rating good good good NG good fair good NG fair Spin performance Spin rate 5,311 5,372 5,445 5,813 4,537 5,397 4,611 5,783 5,352 on approach shots (rpm) Feel Rating good good good good good NG NG good good Durability Rating good good good good NG good NG good good

The following observations are based on the test results in Table 5.

In Comparative Example 1, the cover was soft. As a result, the spin rate on full shots rose and a good distance was not obtained.

In Comparative Example 2, the cover was hard. As a result, the curability to cracking on repeated impact was poor

In Comparative Example 3, the intermediate layer was hard. As a result, the ball had a hard, unpleasant feel on full shots.

In Comparative Example 4, the intermediate layer and the cover were hard. As a result, the ball had a hard feel and the durability to cracking on repeated impact was poor.

In Comparative Example 5, the intermediate layer was soft. As a result, the spin rate on full shots was high and a good distance was not obtained.

In Comparative Example 6, the core did not have an optimal hardness profile. As a result, the spin rate on full shots was high and a good distance was not obtained.

Japanese Patent Application No. 2015-209568 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, a cover, and at least one intermediate layer therebetween, wherein the value obtained by subtracting a surface hardness of an intermediate layer-encased sphere from a surface hardness of the ball, expressed in terms of Shore D hardness, is from 7 to 15; the value obtained by subtracting a surface hardness of the core from the surface hardness of the intermediate layer-encased sphere, expressed in terms of Shore D hardness, is within ±6; in a core hardness profile, the JIS-C hardness at a center of the core is 55±5, the JIS-C hardness at a position 5 mm from the core center is 57±5, the JIS-C hardness at a position 10 mm from the core center is 57±5, the JIS-C hardness at a position 15 mm from the core center is 70±5, and the JIS-C hardness at the core surface is 79±5; the value obtained by subtracting the core center hardness from the core surface hardness, expressed in terms of JIS-C hardness, is at least 22; and letting V be the initial velocity (m/s) of the ball and H be the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N, the value V/H is from 18 to 24 m/s·mm⁻¹.
 2. The golf ball of claim 1, wherein the value obtained by subtracting the surface hardness of the intermediate layer-encased sphere from the surface hardness of the ball, expressed in terms of Shore D hardness, is from 8 to
 13. 3. The golf ball of claim 1 wherein, in the core hardness profile, the JIS-C hardness at the core center is 55±3, the JIS-C hardness at a position 5 mm from the core center is 57±3, the JIS-C hardness at a position 10 mm from the core center is 57±3, the JIS-C hardness at a position 15 mm from the core center is 70±3, and the JIS-C hardness at the core surface is 79±3.
 4. The golf ball of claim 1 wherein, in the core hardness profile, the JIS-C hardness at the core center is 55±2, the JIS-C hardness at a position 5 mm from the core center is 57±2, the JIS-C hardness at a position 10 mm from the core center is 57±2, the JIS-C hardness at a position 15 mm from the core center is 70±2, and the JIS-C hardness at the core surface is 79±2.
 5. The golf ball of claim 1, wherein the core hardness profile satisfies the conditions: 0<C10−Cc≦8,  (1) C10−Cc<Cs−C10, and  (2) 15<Cs−C10,  (3) where Cc is the JIS-C hardness at the core center, C10 is the JIS-C hardness at a position 10 mm from the core center, and Cs is the JIS-C hardness at the core surface.
 6. The golf ball of claim 1 which satisfies the condition: PS ₇ /S/H×100≧6.20 (mm⁻¹),  (4) where PS₇ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface when the ball is subjected to a load of 6,864 N, S is the hypothetical planar area (mm²), defined as the area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N.
 7. The golf ball of claim 1 which satisfies the condition: PS ₂ /S/H×100≦1.85(mm⁻¹),  (5) where PS₂ is the pressed area (mm²), defined as the area of the golf ball that comes into contact with a flat surface when the ball is subjected to a load of 1,961 N, S is the hypothetical planar area (mm²), defined as the area of a cross-sectional circle along the ball diameter were the surface of the ball to be entirely free of dimples, and H is the deflection (mm) of the ball when compressed under a final load of 1,275 N from an initial load of 98 N. 